<html xmlns="http://www.w3.org/1999/xhtml">
<head>

<script>
function foo()
{
  document.getElementById("mtr").appendChild(document.createTextNode("a"));
}
</script>
</head>

<body onload="foo()">

<div><math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
<mrow>  <mfenced open="[" close="]">    <mtable>      <mtr id="mtr">         <mtd><mi>x</mi></mtd>         <mtd><mi>y</mi></mtd>      </mtr>    </mtable>  </mfenced></mrow>
</math></div>

</body>

</html>